1. Field of the Invention
The present invention relates to an improvement of a microwave-band frequency conversion circuit constituted by strip lines.
2. Description of the Prior Art
FIG. 2 is a circuit diagram showing an example of a conventional microwave-band frequency conversion circuit. In FIG. 2, an input terminal 1 is connected to a band pass filter 3 through a line 2, and an output of the band pass filter 3 is connected to a balanced-type frequency converter 7 through a series connection of a line 4, a coupling capacitor 5, and a line 6. The lines 2, 4, and 6 as well as lines constituting the band pas filter 3 and the balanced frequency converter 7 are constituted by strip lines which are formed on a dielectric substrate by printing, photo-etching, or the like. The band pass filter 3 is constituted by a plurality of U-shaped lines, for example, three U-shaped lines, as shown in FIG. 2, each having a wavelength substantially one-half of that of an input signal, the band pass filter being arranged such that the respective opening portions of the U-shaped lines are alternately reversed in orientation and that respective opposite sides of the U-shaped lines are disposed in parallel to each other. In the balanced frequency converter 7, respective lines 9 and 10 each having a wavelength substantially one-quarter of that of a local oscillation signal are connected to the opposite ends 8a and 8b of a U-shaped line 8 having a wavelength substantially one-half of that of the inoput signal and extended in the directions of the respective extensions of the opposite sides of the U-shaped line 8. Further, two mixing diodes 11 and 12 are disposed in series to each other between the opposite ends 8a and 8b of the U-shaped line 8, the junction between the diodes 11 and 12 being connected to one end of each of the capacitor 13 and a coil 14. The respective other ends of the capacitor 13 and the coil 14 are connected to a local oscillation signal input terminal 15 and an intermediate-frequency signal output terminal 16, respectively.
In the thus arranged circuit, among input signals supplied to the input terminal 1, only the input signal having a frequency in a specific frequency band is selected owing to the resonance of the lines constituting the band pass filter 3, and supplied to the balanced frequency converter 7 through the line 4, the coupling capacitor 5, and the line 6. The input signal supplied to the one end 8a of the U-shaped line 8 of the balanced frequency converter 7 is produced from the other end 8b of the same with a phase inverted by the U-shaped line 8. Accordingly, a potential difference is generated between the opposite ends 8a and 8b of the U-shaped line 8 in accordance with the input signal, and supplied to the diodes 11 and 12. A local oscillation signal externally applied to the local oscillation signal input terminal 15, on the other hand, is supplied to the diodes 11 and 12 through the capacitor 13. The lines 9 and 10 are made to resonate relative to the local oscillation signal so as to cause the local oscillation signal to easily flow into the diodes 11 and 12. Then, the local oscillation signal and the input signal are mixed with each other by the diodes 11 and 12 so as to form an intermediate-frequency signal. The capacitor 13 is selected to have impedance which is low with respect to the local oscillation signal while high with respect to the intermediate-frequency signal so that the intermediate-frequency signal is prevented from flowing into the local oscillation signal input terminal 15. The coil 14 is, on the contrary, selected to have impedance which is high with respect to the input signal and the local oscillation signal and while low with respect to the intermediate-frequency signal so that the input signal and the local oscillation signal are prevented from flowing into the intermediate-frequency signal output terminal 16. Accordingly, only the intermediate-frequency signal is produced from the intermediate-frequency signal output terminal 16.
FIG. 3 is a circuit diagram showing a frequency conversion circuit which has been conventionally proposed for the purpose of further miniaturizing the conventional frequency conversion circuit having such a structure as shown in FIG. 2. In FIG. 3, circuit components corresponding to those in FIG. 2 are correspondingly referenced and repeated explanation is omitted.
In FIG. 3, the frequency conversion circuit is different from that shown in FIG. 2 in that one of the opposite sides of a U-shaped line of a band pass filter 3 is disposed adjacently to and in parallel with one of two opposite sides of a U-shaped line 8 of a balanced frequency converter 7 so as to provide inductive coupling between the band pass filter 3 and the balanced frequency converter 7. The U-shaped lines of the band pass filter 3 disposed adjacently to the U-shaped line 8 of the balanced frequency converter 7 are arranged such that the respective openings of U-shaped are alternatively reversed in orientation.
In the foregoing conventional frequency conversion circuit having the structure as shown in FIG. 2, it is necessary to connect the band pass filter 3 to the balanced frequency converter 7 through the line 4, the coupling capacitor 5, and the line 6 to sufficiently separate the band pass filter 3 from the balanced frequency converter 7 so as not to provide inductive coupling therebetween. This is because, since the band pass filter 3 is connected to the balanced frequency converter 7 through the lines 4 and 3, it is necessary to provide the coupling capacitor 5 for the purpose of DC blocking, so that the phase of an input signal transmitted from the band pass filter 3 to the balanced frequency converter 7 through the line 4, the coupling capacitor 5, and the line 6 is different from that of the input signal transmitted from the former to the latter through inductive coupling and it is necessary to prevent the input signal from being transmitted through inductive coupling. Accordingly, there has been such a problem that it was required to prepare a larger space for providing the whole arrangement of the frequency conversion circuit because the distance between the pass filter 3 and the balanced frequency converter 7 had to be selected to be not smaller than five times as long as the conductor width of the line. Accordingly, the conventional frequency conversion circuit shown in FIG. 2 is unsuitable for the purpose of miniaturization of the device in which the frequency conversion circuit is incorporated.
In the conventional frequency conversion circuit having a structure as shown in FIG. 3, on the other hand, there has been such an advantage that not so large space is required for providing the band pass filter 3 and the balanced frequency converter 7 because the former is connected to the latter through inductive coupling. However, since the diodes 11 and 12 are connected between the opposite ends 8a and 8b of the U-shaped line 8 of the balanced frequency converter 7, the impedance between the opposite ends 8a and 8b is low and acts as a load for an induced voltage induced in the U-shaped line 8 by an electromagnetic wave radiated from the band pass filter 3, so that the inductive coupling between the band pass filter 3 and the balanced frequency converter 7 is weakened. Therefore, there has been such a problem that the intermediate-frequency signal to be produced was small because a sufficiently large input signal could not be obtained in the balanced frequency converter 7.